Re: [RFC] Track mlock()ed pages

[Andrew Morton <akpm@osdl.org>]

On Sat, 27 Jan 2007 17:19:21 -0500
Rik van Riel <riel@redhat.com> wrote:

> Andrew Morton wrote:
> 
> > Of course it would.  But how do you know it is "too expensive"?  We "scan
> > all the vmas mapping a page" as a matter of course in the page scanner -
> > millions of times a minute.  If that's "too expensive" then ouch.
> 
> We can do it lazily.
> 
> At mlock time, move pages onto the mlocked list, unless they
> are there already.

Needs another page flag to determine what list the page is on (eek).

> On munlock, move pages to the active list.

We'd need to determine whether some other vma has mlocked the page too. 
That's either the page_struct refcount or the vma walk.  The latter is
equivalent to what I'm suggesting.

>  For mlock-only
> memory (shared memory segments?) we could add a simple check
> to see if the next process on the list has the page mlocked,
> checking only that one.
> 
> While scanning the active list, move mlocked pages that are
> found back onto the mlocked list.
> 
> This lazy movement of pages will impact shared libraries,
> but probably not shared memory segments.
> 
> Does this sound workable?

I'm still not sure what problem we're trying to solve here.

Knowing how many mlocked pages there are in a zone doesn't sound terribly
interesting and I don't recall ever wanting to know that.

Being able to keep mlocked pages off the LRU altogether sounds more useful.

It's all rather a tight corner case - people don't use mlock much.